1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copier and an electrophotographic printer.
2. Description of Related Art
For color printers, toners of four colors C, M, Y and K are normally prepared. The four colors C, M, Y and K are C (cyan), M (magenta) and Y (yellow) which are the three primary colors, and K (key plate, that is, black). Although black can be expressed by the three colors C, M and Y in theory, the mixture of C, M and Y cannot reproduce complete black in actuality. For this reason, not the mixture of the three colors C, M and Y but the single color K is used when black is expressed in printing.
Color printers are provided with an image process device for executing an image forming process, for each color. The image process device is provided with a photoreceptor (electrostatic latent image carrier), a charging unit, an exposing unit, a developing unit, a cleaner (cleaning blade), a transfer roller and the like as image process parts.
A color printer described in Patent Reference 1 is intended to prevent the above-mentioned image process parts for each color from deteriorating because of low frequencies of use of toners of C, M and Y. Specifically, the following situations are assumed: For example, when C, M and Y are used for a part of a printed image constituted mainly by black and white, the frequencies of use of C, M and Y are low compared with the frequency of use of K. However, when a color image is printed, the image process devices of the four colors are driven at the same time. At this time, the image process devices for C, M and Y are driven under a condition where the toner supply amount is small. When the amount of toner supplied to the photoreceptor is small, for example, the friction between the photoreceptor and the cleaning blade is high. Consequently, the cleaning blades for C, M and Y are damaged.
Accordingly, in the color printer described in Patent Reference 1, first, the frequency of use of each color is identified based on the amount of image information of each color included in the printed image. Then, when the frequency of use of at least one of C, M and Y is low, in the color printer, the toners of C, M and Y are mixed into the toner of K to an extent that the hue of black is not changed. By consuming the toners of C, M and Y in this way, the supply amounts of the toners of C, M and Y are increased.    [Patent Reference 1] Japanese Unexamined Patent Application Publication No. 2002-6582.
In printing processing, toner is supplied from the developing unit to the photoreceptor. The toner in the developing unit is held at a fixed amount. For this reason, toner is supplied from a toner tank to the developing unit so as to make up for the consumed toner. During the execution of printing processing, toner is being stirred in the developing unit. As the stirring time increases, toner is damaged. That is, deteriorated toner occurs. However, as long as toner is properly discharged from the developing unit by printing processing, the deteriorated toner is restrained from being accumulated in the developing unit.
When the density of the printed image is low, although the toner is stirred as usual, the toner discharge amount is smaller than normal. For this reason, the deteriorated toner that occurs by the stirring is not properly discharged. Consequently, the deteriorated toner is accumulated in the developing unit.
The deteriorated toner degrades the quality of printed images. Therefore, it is necessary to forcibly discharge the deteriorated toner in order to prevent the deteriorated toner from being accumulated in the developing unit in an amount not less than the fixed amount. The forcible discharge of the deteriorated toner is performed, for example, by supplying toner corresponding to solid printing from the developing unit to the photoreceptor. The toner supplied to the photoreceptor is removed by cleaning.
When the forcible discharge of toner is executed, normal toner is also consumed as well as the deteriorated toner. Consequently, the toner cartridge life is reduced. If the forcible discharge of toner is executed during the execution of continuous printing, continuous printing is intermitted. Consequently, the productivity of printing processing is reduced.